It is often desired to lower the sound pressure level in rooms through absorption. In rooms where both speech and music occur, this absorption should preferably give the same resonance time over the entire range of frequencies area of interest and which can be from 50-5000 Hz. This is normally attempted by a combination of different materials with varying sound absorption coefficients in varying frequency intervals. Generally, two different types of sound absorbents are possible in this connection, namely those that are of porous material which are effective from a few hundred Hz and upward, or so called hard absorbents which give high absorption at low frequencies, but are not effective at high frequencies. The absorption coefficient for an absorbent with low resonance frequency is normally not high, which demands that large areas are covered with absorbents to lower the resonance time. To meet the demands on resonance time in rooms such as classrooms, and which may not exceed 0.6 seconds in valid frequency range, an additional absorption surface is needed, which generally covers the entire ceiling. This in its turn results in a very poor accoustic surrounding. Since it is the lowest frequency range that determines the size of the additional absorption surface area, it has been natural to try to increase actual absorption of the material in these frequency ranges.
It is known that porous absorbents of mineral wool or similar materials get an improved low frequency absorption if mounted as an inner ceiling with a distance to the existing ceiling. The distance decides to a large degree how far down in frequency that sound is effectively absorbed. With a distance of e.g. 30 cm a reasonably good absorption down to approximatly 300 Hz is achieved. There are however limits as to how low an inner ceiling can be mounted and for practical reasons lowering of absorption to below 250 Hz has not been possible. An absorbing inner ceiling gives maximum absorption at that frequency which coincides with a quarter wavelength between the absorbent and the existing ceiling.
It is also well known that a long resonance time negatively affects speech comprehension in rooms of different kinds and to lower resonance time acoustic absorbers of different kinds have been introduced. The most common method is still to cover all or part of a room's ceiling with absorbents. A complete inner ceiling however absorbs even the early reflections which are needed for speech comprehension in the rear of the room, whereas it is true that a partially covered inner ceiling with reflecting surfaces in the center, aids the early reflections to reach the rear of the room, but gives a poor absorption. Both these methods of arranging the sound absorbents have minimal absorption under 200 Hz.